According to TMZ, Selena was worried that a burglar was inside her house when she returned home around midnight and noticed an open back door. She immediately called the police and they rushed to her residence.
Fortunately, the cops didn't find anyone in her house. They found no evidence of forced entry. It's possible she left the door open herself, but she's reportedly still freaked out.
All of this comes after a random man showed up to her a house a few months ago...while she was home. Read about that here. She probably thought it was him again? Creepy.